350 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



adult forms of the first live in the air passages, but in the larval 

 stage infest the internal organs. The acari are not all parasitic 

 Those which are, however, almost invariably inhabit the surface of 

 the body. The order includes the ticks. 



The insects are air-breathing and are provided with a trachea. 

 Their bodies are divided into three distinct parts the head, thorax 

 and abdomen. The head is provided with a pair of antennae, the 

 thorax with three pairs of limbs and frequently with two pairs of 

 wings. The order includes seven or eight sub-orders, only two of 

 which furnish parasites the Dipteria and Hemipteria nearly all 

 the species of which spend the most of their life on the surface, or 

 within the substance of the skin; at least in their adult stage. 



Parasites live on the surface of the body or in organs. The 

 first are called ectoparasites and the second endoparasites. Each 

 division will be discussed in its order. 



The life history of parasites in general is quite simple, but 

 when the life history of those which act as parasites during but a 

 portion of their life is considered, we meet with a variety of hab- 

 its. Some are parasitic only in the adult stage, and even in this 

 stage may spend all or only a portion of the time on the host. 

 Others are parasitic only in the larval stage, during which time it 

 is a constant parasite, leaving its host only to enter the pupa stage. 

 For example, the mosquito, horse flies, and gnats are parasitic only 

 in the adult form, the gnat and mosquito passing the larval and 

 pupa stages in moist places or in water. On the other hand, the 

 larvae of the screw worm fly and bot-fly are parasitic, while the 

 adult is not. The time required for the different species of para- 

 sites to pass through the different stages of their life varies with 

 each family or species. Some require months, while others require 

 but a few days. In a very large number of animal parasites their 

 entire life is passed upon, or within, the host. Of the internal 

 parasites, the larvae are frequently migratory and inhabit organs or 

 tisstfes very remote from those in which the adult is found, and in 

 different species of animals. For instance, the larvae of a common 

 tape-worm of man are found in the flesh of cattle, and another in 

 the flesh of swine, in the latter constituting the so-called pork 

 measles. 



It has been argued by some that insects which now act as para- 

 sites have been forced to dp so by chance, or certain circumstances, 

 and, although now parasitical, are the descendants of non-parasitic 

 insects. There are numerous facts upon which deductions are 

 founded, especially from a biological standpoint, and by which 

 the theory of evolution must be accepted to a certain degree, since 

 it is not reasonable to suppose that a given form of parasite existed 

 before the existence of its host. In other words, it must be con- 

 sidered that the force of habit has, in many instances, transformed 

 a non-parasitic insect into a parasitic one, in which certain organs 

 have become more highly developed, while others have degenerated. 

 The natural tendency for an insect once started in a parasitic life is 

 to become more and more parasitic, hence the transformation of 



